Generally, fuel cells use high-efficiency clean electricity generation technology in which oxygen in the air and hydrogen contained in a hydrocarbon material, such as natural gas, coal gas, methanol, etc., are directly converted into electric energy by an electrochemical reaction. According to the kind of electrolyte, fuel cells are classified into an alkali fuel cell, a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell and a polymer electrolyte fuel cell.
The solid oxide fuel cell (SOFC), all components of which are solid, is operated at a high temperature ranging from 600° C. to 1000° C. Among the several types of existing fuel cells, the SOFC has the highest efficiency and the lowest pollution rate. In addition, the SOFC has other several advantages in which a fuel reformer is not required, and it can be easily used in a combined electricity generation system. Further, the SOFC can be used as a high-temperature solid oxide electrolyzer cell (SOEC) by performing an inverse electrochemical reaction.
Electrochemistry reaction devices, such as the solid oxide fuel cell, the high-temperature electrolyzer cell, etc., are classified into a flat type and a cylindrical type, according to the shape. The flat type electrochemistry reaction device has the advantage of high power density (output) but is disadvantageous in that the area of a portion to be sealed for gas is comparatively large, there is a thermal shock that occurs because of the difference in coefficients of expansion between components when stacked, and it is difficult to greatly increase the size thereof. The cylindrical type electrochemistry reaction device has high resistance to heat stress and high mechanical strength and can have a large size because it is manufactured by extruding. However, the cylindrical type is disadvantageous in that power density (output) is low.
Representative examples of flat-tubular electrochemistry reaction devices (for example, flat-tubular solid oxide fuel cells) that take advantage of the flat type electrochemistry reaction device and the cylindrical type electrochemistry reaction device were proposed in Korean Patent Laid-open Publication No. 2005-0021027 and US Patent No. 2003-0224240A1. Flat-tubular electrochemistry reaction devices have a stack structure in which cells are stacked one on top of another to enhance the output. However, there is a difficulty in collecting current at the anode and cathode sides. Further, the number of gas flow manifolds increases in proportion to the number of cells, and it is not easy to reliably seal the stack structure to prevent gas from leaking.
Meanwhile, a flat-tubular electrode support and a unit cell for a solid oxide fuel cell were proposed in Korean Patent Laid-open Publication No. 2009-0084160. A cell stack using the electrode support and the unit cell was proposed in Korean Patent Laid-open Publication No. 2009-0104548.
However, in the conventional electrochemistry reaction devices (the flat-tubular solid oxide fuel cell and the flat-tubular high-temperature electrolyzes cell), a cell stack is configured in such a way that metal connection plates are formed in semi-arc shapes or planar shapes, and ceramic cells are seated on the corresponding metal connection plates. Therefore, due to a difference in a coefficient of expansion between the ceramic cells and the metal connection plates, the cells may be damaged. Further, the metal connection plates may be oxidized by making contact with air, thus reducing the current collection performance.
In addition, in the conventional electrochemistry reaction devices (the flat-tubular solid oxide fuel cell and the flat-tubular high-temperature electrolyzer cell), manifold portions are sealed to isolate an oxidizer (air or oxygen) supply part from a reducer (hydrogen or hydrocarbon) supply part. When cells are stacked one on top of another to enhance the output, the number of gas flow manifolds increases in proportion to the number of stacked cells. Because the shape of each manifold portion to be sealed is complex, it is not easy to reliably seal the gas. Moreover, it is difficult to determine a gas sealing structure and a sealing material, because the operating temperature is comparatively high.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a flat-tubular solid oxide cell stack which is configured such that the stress of a cell stacking structure is minimized, a portion to be sealed is minimized, the length of a path for a chemical reaction is increased, the efficiency with which electricity is generated is enhanced when it is used as a fuel cell, and the purity of generated gas (hydrogen) is increased when it is used as a high-temperature electrolyzer cell.